Pregnancy is characterized by seemingly endless physiological changes — changes in immune function, energy storage, blood volume, and hormone production, all of which are necessary to support the start and development of a healthy pregnancy.
In this post, we’ll go over some of the hormone-related changes that occur in the first couple of weeks after implantation, covering some key hormones you may have heard about like estradiol, progesterone, and hCG, and some you may not have heard as much about (relaxin, anyone?).
Hormones after ovulation, sans implantation
During ovulation, your ovarian follicle releases a developed egg for possible fertilization. That follicle then forms a structure called the corpus luteum, which pumps out two hormones into your body: progesterone (P) and estradiol (E2):
- Progesterone helps prep the uterine lining (aka endometrium) for pregnancy, but if an egg isn’t fertilized, you’ll experience a rapid drop in P. That’s why some of us get periods: P drops, your uterine lining can’t be maintained, and it sheds during menstruation. (This is why some doctors prescribe high doses of P to "kick-start" periods if you haven’t had one in a while.)
- E2 produced by the corpus luteum follows a similar pattern: it rises after ovulation, then falls if an egg doesn’t get fertilized.
In a cycle where implantation doesn’t happen, the corpus luteum will die off about two weeks after ovulation. The corpus luteum disappearing is what contributes to dropping levels of P and E2 at the end of your cycle, which trigger menstruation.
How your hormones change after implantation
In the case of a successful implantation (which usually happens about a week after fertilization), rather than P and E2 dropping about two weeks after ovulation and causing the endometrium to shed its lining, these hormones continue to rise.
We see these continual rises in P and E2 because the corpus luteum (which produces these hormones) stays put if implantation occurs — and it continues to be the main source of P and E2 production until the beginning of the second trimester. The endometrium provides a place for the embryo to begin developing, and some endometrial cells develop into part of the placenta: the key structure that provides nourishment and gets rid of waste for a developing fetus.
Because patterns of P and E2 are largely similar in the two-ish weeks after ovulation regardless of whether implantation happened or not, symptoms related to increases in these hormones can be indicative of a normal luteal phase, or of an early pregnancy — meaning they can’t really tell you much.
Within one day of implantation, another super important hormone makes its debut: human chorionic gonadotropin (hCG). After implantation, a structure called the chorion (that forms around the embryo) begins producing hCG (hence the name: human chorionic gonadotropin) and eventually develops into part of the placenta.
At-home pregnancy tests are based on this foundation. They compare urine hCG levels to a predetermined hCG threshold that indicates pregnancy (most frequently between 10 and 25 mIU/mL). Though both false positives and false negatives can happen, they’re only seen in rare cases. If someone was on medications that directly affect hCG, or if there was very recently a chemical pregnancy or miscarriage, positive pregnancy tests in either of those cases would be false positives.
Early on in pregnancy, hCG levels increase roughly 50% every day, and some research suggests a relationship between hCG and symptoms like nausea and vomiting during the first eight weeks of pregnancy (which about 70-80% of pregnant people report experiencing). The majority of people who do experience these symptoms will report them being resolved by the beginning of the second trimester, which is also when hCG starts to decrease.
Why? hCG’s main job (other than telling you whether or not you’re pregnant!) early on is to maintain the corpus luteum — that ruptured follicle that pumps out P and E2 to maintain the endometrium (which is necessary for a developing pregnancy). Once the second trimester rolls around, though, the placenta is developed and active enough to produce sufficient amounts of P and E2, meaning the corpus luteum’s services are no longer needed. As the corpus luteum naturally fades away, so does hCG.
Another sign or symptom linked to early pregnancy is something called implantation bleeding, which happens to about 30% of pregnant people. Implantation bleeding is not a symptom of changing hormones, but rather a direct result of an embryo implanting into the uterine lining. If the embryo ruptures some blood vessels during the implantation process, the result will be some light spotting.
What about AMH, LH, and FSH in early pregnancy?
Though anti-Mullerian hormone (AMH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) are crucial when it comes to understanding ovarian reserve and ovulation, they’re not so exciting early on in pregnancy. The primary roles of LH and FSH are to nudge along the development of ovarian follicles and to cause those follicles to rupture during ovulation.
Soon after implantation and early on in pregnancy, the brain regions responsible for reproductive hormone production (mainly, areas called the hypothalamus and the pituitary gland) shift their focus from producing hormones that promote follicular growth and ovulation over to those that maintain a developing pregnancy — i.e., processes that LH and FSH don’t play much of a role in. We therefore see low LH and FSH levels across pregnancy.
Similarly, as AMH is a direct product of developing follicles and follicular development is put on the back burner during pregnancy, AMH decreases during pregnancy, but rebounds soon after birth. Because there haven’t been many longitudinal studies looking at AMH levels at ovulation, implantation, and across pregnancy, we aren’t completely sure exactly when or how quickly AMH starts decreasing after implantation.
Just for fun: The role of relaxin
Though the hormone relaxin hasn’t received as much research attention as P, E2, and hCG when it comes to understanding the hormones at play during early pregnancy, there are several lines of evidence suggesting relaxin does play a role in the development and function of the endometrium by promoting things like blood vessel formation. But relaxin might be better known for its role in something completely different: its ability to facilitate childbirth.
Across pregnancy, levels of relaxin steadily increase and are directly involved in the loosening of ligaments, muscles, and joints (meaning that if you’re feeling a little more flexible during pregnancy, this is the hormonal culprit). Though relaxin can have an effect everywhere in the body, arguably its most important effects are on the uterus and pelvis. As the developing fetus and placenta grow, relaxin signals the uterine ligaments to loosen up — allowing the uterus to expand.
During childbirth, relaxin gets the ligaments connecting the left and right halves of our pelvic bones to loosen up too, allowing for the space between those bones to expand — making it a little easier for the baby to get through. (And if you thought your hips seemed a little wider after giving birth, you weren’t imagining it — the joints separating a bit is another byproduct of pregnancy.)
Just like what happens with P and E2, patterns of relaxin in the two-ish weeks after ovulation are the same regardless of whether or not implantation happened — in cycles where implantation does not occur, relaxin still increases across the luteal phase and drops around menstruation. So, if you notice any symptoms suggestive of increased relaxin, it wouldn’t be possible to tell if they’re because of normal variation in the luteal phase or because of a developing pregnancy.
Test early for pregnancy with Modern Fertility
Though the few weeks after fertilization are associated with changes that gear your body up for a developing pregnancy, the lack of symptoms that are specific to pregnancy this early on (i.e., symptoms experienced during early pregnancy and not during a luteal phase where implantation didn’t occur) are few and far between.
That’s why the best way to detect an early pregnancy is through measuring hCG through a pregnancy test, like Modern Fertility’s Pregnancy Test. It's 99% accurate* from the day of your missed period.
What makes our Pregnancy Test extra special:
- You can start testing for pregnancy as early as 6 days before your missed period.
- Our test is more affordable than the leading pregnancy tests, based on the average price of the top 5 pregnancy tests in retail stores by sales volume. (Source: Nielsen retail sales data in 2021.)
- It comes with easy-to-understand, clinically sound instructions that demystify the chemistry behind testing for pregnancy (although you should already be a pro after getting through this article!).
*Accuracy is based on detecting typical pregnancy hormone levels – note that hormone levels may vary. See insert for more details.
This article was reviewed by Dr. Jennifer Conti, MD, MS, MSc. Dr. Conti is an OB-GYN and serves as an adjunct clinical assistant professor at Stanford University School of Medicine.